endnotes.heading.groups — Display the endnotes by groups
      <xsl:param name="endnotes.heading.groups"/>
In the endnote section, the notes are grouped according to the level
    defined by the elements listed in this parameter. Each group has a heading
    that references the element that contains these notes.
    By default, dblatex can group per chapter or per
    part. For more details
    see the section called “Grouping Endnotes”.